1. Field of the Invention
The present invention relates to an abnormality diagnosing apparatus and method for an air-fuel ratio sensor system of an internal combustion engine for diagnosing an air-fuel ratio sensor used for detecting an air-fuel ratio of mixture supplied to an engine.
2. Description of Related Art
An air-fuel ratio feedback control system of an internal combustion engine does not function correctly when there is an abnormality in an air-fuel ratio sensor for detecting the air-fuel ratio of air-fuel mixture supplied to the engine. It is thus necessary to detect timely an abnormality occurring in the air-fuel ratio sensor system.
For this reason, JP-A 1-232143 proposes to use a temperature sensor for detecting the temperature of the air-fuel ratio sensor. According to this technology, if the temperature of the air-fuel ratio sensor detected by the temperature sensor does not increase to a predetermined value, a heater for activating the air-fuel ratio sensor is determined to be abnormal. This not only requires a temperature sensor for detecting the temperature of the air-fuel ratio sensor, but also causes a disadvantage that it is impossible to detect failures other than those occurring in the heater.
In addition, JP-A 3-189350 proposes an apparatus for controlling electric power supplied to a heater to adjust the resistance of the heater to a target value. When the amount of electric power supplied to the heater rises beyond a predetermined range, the target value of the heater resistance is determined to be abnormal. The target value of the heater resistance generally becomes abnormal when the battery or the sensor is replaced, that is, during repair work or inspection. Thus, the heater control apparatus is not capable of determining the reliability of the air-fuel ratio sensor in the course of air-fuel ratio feedback control.
Further, JP-A 9-4494 proposes an abnormality diagnosing apparatus. It is, however, impossible to detect a circuit-shorting or a breakage in the air-fuel ratio sensor system in some cases.